Urokinase (UK, urinary plasminogen activator) excretion (UKE) was assayed by radiochemical esterolytic assay in human urine. In normal volunteers, upright posture increased UKE and decreased urine kallikrein excretion and urine flow, when compared to supine posture. This suggested a possible relationship of UKE with sympathetic nervous system activity. No effect on UKE was observed during dietary variation of sodium and potassium, or during infusion studies with vasoactive amines. No diurnal variation of UKE was observed. The presence of mucoprotein and the formation of cold precipitates which change UK in many urines necessitates assay of fresh samples. The origin of urinary kinins was investigated using chromatography and bioassay. Human urinary kallikrein generated lys-bradykinin and bradykinin. Met-lys-bradykinin (MLBK) is largely artifactual, resulting from the action of uropepsin on urokininogen in acidified urine. MLBK is not found in urines to which the propepsin inhibitor pepstatin has been added.